Cilia are micro-scale, hair-like structures that exist in nature and extend from an organism's cells. Fish, for example, utilize cilia to assist in performing a variety of functions, such as preying or avoiding danger. In particular, when the cilia are exposed to a change in fluid flow, they bend and transmit a signal to the organism, providing the organism with sensory information enabling an appropriate reaction.
Recently, artificial cilia have been developed that could be used for various applications such as microfluidic propulsion, flow sensing, or the study of cell mechanics. For the flow sensing applications, different techniques have been utilized to detect cilia bending due to flow, such as by using thermal, piezoresistive, piezoelectric and magnetic bases.
For instance, a piezoresistive hair flow sensor, described in Chen et al. “Design and Characterization of Artificial Haircell Sensor for Flow Sensing With Ultrahigh Velocity and Angular Sensitivity,” J. Microelectromech. Syst., 2007: 999-1014, consists of a 600 μm by 80 μm vertical SU-8 hair-like structure fabricated by photolithography and a silicon resistor. This sensor was apparently able to detect constant air flow between 0 to 20 m/s with a sensitivity of 100 mm/s and water flow from 0 to 0.4 m/s with a sensitivity of 5 mm/s. The sensor was also apparently able to detect alternating flow velocity amplitudes down to the order of 0.7 mm/s in water at a frequency of 50 Hz.
Flow sensors based on a vertical cilium and a strain gauge were discussed by Chang Liu in “Micromachined biomimetic artificial haircell sensors,” Bioinspiration and Biomimetic, 2007, 2 S162. This study included two prototypes: silicon-based and polymer-based cilia. The silicon-based sensor was mounted on a glass plate and placed in a water tunnel with laminar flow. For water flows with velocities from 0 to 1 m/s, a sensitivity of 0.5 mm/s was reported. The response of the polymer-based sensor increases exponentially within the tested range, when applying air flow with velocities ranging from 0 to 30 m/s. Chang concluded that silicon-based flow sensors showed higher sensitivity, whereas polymer-based sensors were more robust.
Hein et al. proposed an inorganic nanocilia sensor based on magnetic nanowires (NWs) in “Fabrication of BioInspired Inorganic Nanocilia Sensors,” IEEE Transactions on Magnetics, 2013, 49, 191. The sensor utilizes the magnetic stray field of cobalt NWs for a biomimetic sensing approach. The NWs were mounted on a giant-magneto-resistive sensor to detect their motion. The sensor had two possible applications: flow sensing and vibration sensing. Water flows were detected from 3.3 m/s to 40 m/s with a sensitivity of 0.55 μV/m/s and a signal to noise ratio of 44, and vibrations in the low earthquake-like frequency range of 1 to 5 Hz. The stiffness of the bare magnetic NWs prevents the measurement of low flow velocities. Nanocilia made of metals like Cobalt have high possibility of corrosion, limiting their use for applications in, for example, microfluidic devices. Magnetic polymer cilia have also been realized using superparamagnetic nanoparticles embedded in thin polymer films for various applications. This approach requires the application of rather large magnetic fields. For instance, Khaderi et al. “Magnetically-actuated artificial cilia for microfluidic propulsion,” Lab on a Chip, 2011, 11, 2002, applied a rotational magnetic field of 115 mT in amplitude and Digabel et al. “Magnetic micropillars as a tool to govern substrate deformations,” Lab on a Chip, 2011, 11, 2630, used 23×103 T·m−1 magnetic field gradient to actuate the cilia. A favorable property of magnetic cilia is the absence of an electric contact and the possibility of remote detection or actuation.
Recently, there has been a great interest in developing sensors with low power consumption. However, reducing power consumption usually leads to a reduction in the resolution. For example, a low power thermal flow sensor described in Cubukcua et al. “A 2D thermal flow sensor with sub-mW power consumption,” Sensors and Actuators A: Physical, 2008, 2, 142, produced a resolution below 10 mm/s at 177 μW. However, high resolution thermal flow sensors have a power consumption of more than 1 mW. In this regard, hair flow sensors are attractive options and have been shown to operate at power consumption as low as 140 μW providing a resolution of 0.9 m/s.